The present invention relates to a method for monitoring switching paths that can safely connect a load with a supply voltage and safely disconnect the load from the supply voltage, as well as to a switching circuit for safely connecting and disconnecting the load.
For safely connecting a load to and disconnecting a load from a supply voltage or supply voltages, at least two switching paths must be provided, wherein electric power should be supplied to the load only if the load is connected to the supply voltage or voltages through both switching paths. For example, the load of machine tools, production machines and other electrical machines is generally disconnected from all supply voltages simultaneously, which is typically referred to as “Safe Stop.”
The switching paths must be tested from time to time to ensure their proper operation. Safety standards and agency regulations for electric and electronic systems therefore require periodic tests, for example, every couple of hours, for example every eight hours.
Conventional systems and methods, such as described, for example, in German patent publication no. DE 199 51 957 A1, involve testing of the operation of the switching paths by polling the status of the switching paths sequentially, i.e. with a time offset. A monitoring device measures a (single) state signal, based on which the monitoring device determines the functionality of the switching paths. This conventional method suffers shortcomings because the required time offset between the switching paths increases the switching circuit complexity as a consequence of the need for additional and/or more complex switching circuit components. In addition, assurances must be taken to specify the order in which the switching circuits are addressed.
It would therefore be desirable and advantageous to provide an improved method and a corresponding switching circuit for monitoring switching paths that connect a load to a supply voltage and disconnect the load from the supply voltage, which obviates prior art shortcomings and can be implemented without requiring additional and/or more complex switching circuitry.